Mastering LeetCode's Palindrome Challenge: A Guide for Engineering Interviews

Unlock the secrets to solving the Palindrome Verification problem with ease, whether you're an interview veteran or a coding newcomer.


3 min read

Mastering LeetCode's Palindrome Challenge: A Guide for Engineering Interviews

In the realm of software engineering interviews, the ability to dissect and conquer algorithmic challenges is paramount.

Today, I'm delving into a classic yet intriguing problem often encountered on platforms like LeetCode: determining whether a given string is a palindrome, considering only alphanumeric characters and disregarding cases. This problem not only tests your string manipulation skills but also your ability to apply efficient solutions under constraints.

This is LeetCode problem 125: Valid Palindrome.

Introduction to the Palindrome Problem

A palindrome is a word, phrase, number, or other sequences of characters that reads the same forward and backward, ignoring punctuation, case, and spaces. For instance, "A man, a plan, a canal: Panama" is a palindrome because, if we filter out non-alphanumeric characters and ignore case, it reads 'amanaplanacanalpanama', which is the same forwards and backwards.

The challenge lies in efficiently processing the string to ignore non-relevant characters and case differences, providing a solution that's both elegant and optimal in terms of time and space complexity.

Strategy for Solution and Complexity Analysis

The core approach to solving this problem involves two steps: normalization and comparison.

  • Normalization: Convert all characters to the same case (lowercase or uppercase) and remove non-alphanumeric characters.

  • Comparison: Check if the normalized string reads the same forward and backward.

Big O Notation Analysis: The time complexity for the normalization process depends on the length of the string, making it O(n). The comparison, whether we use a two-pointer approach or compare against a reversed string, also operates in O(n) time. Thus, the overall time complexity remains O(n). Space complexity is O(n) as well, due to the additional storage needed for the normalized string.

Python Solutions

Two-Pointer Approach

def isPalindrome(s: str) -> bool:
    def alphaNum(c):
        return c.isalnum()

    # Normalize: lowercase and filter out non-alphanumeric characters
    filtered = ''.join(filter(alphaNum, s.lower()))

    # Two-pointer comparison
    left, right = 0, len(filtered) - 1
    while left < right:
        if filtered[left] != filtered[right]:
            return False
        left, right = left + 1, right - 1
    return True

Functional Approach

def isPalindrome(s: str) -> bool:
    # Normalize: lowercase and remove non-alphanumeric characters
    normalized = ''.join(c.lower() for c in s if c.isalnum())
    # Check palindrome using string reverse
    return normalized == normalized[::-1]

TypeScript Solution

function isPalindrome(s: string): boolean {
    // Normalize: lowercase and remove non-alphanumeric characters
    const normalized = s.toLowerCase().replace(/[^a-z0-9]/g, '');

    // Check if palindrome
    return normalized === normalized.split('').reverse().join('');

Java Solution

public class Solution {
    public boolean isPalindrome(String s) {
        // Normalize: lowercase and remove non-alphanumeric characters
        String normalized = s.toLowerCase().replaceAll("[^a-z0-9]", "");

        // Check if palindrome
        return normalized.equals(new StringBuilder(normalized).reverse().toString());


Tackling the palindrome problem showcases the importance of string manipulation techniques and efficient problem-solving strategies in software engineering interviews.

Whether you choose Python, TypeScript, or Java, the key lies in understanding the problem's nature and applying a suitable approach. Remember, practice and familiarity with these concepts will not only help you ace interview questions but also improve your overall coding prowess.

I hope this guide provides you with a clear roadmap to solving the palindrome challenge and adds a valuable tool to your interview preparation kit. Happy coding, and best of luck on your interview journey!

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